This invention relates to a structure of joining, to bus bars, a chip part connected to an electronic device to be mounted on a circuit board, a J/B (junction box) or the like,.
As shown in FIG. 5, when chip parts 21, 22 and 23 of an electronic device 20 are soldered to bus bars (bar-like metallic conductors) 25, 26, 27 and 28, the chip parts 21, 22 and 23 are placed on soldering lands 25a, 26a, 27a and 28a (indicated in dots-and-dash lines in the drawings) of two or more (four in the illustrated example) bus bars 25, 26, 27 and 28. Then, connecting portions 21t, 22t and 23t of the thus placed chip parts 21, 22 and 23 are joined by soldering to the corresponding bus bars 25, 26, 27 and 28, respectively.
More specifically, in the soldering operation, when molten solder is dropped and supplied to be coated onto each connecting portion 21t, 22t, 23t of the chip part 21, 22, 23 and the associated soldering land 25a, 26a, 27a, 28a of the bus bar 25, 26, 27, 28, the molten solder forms a wide wetting area on the surface of the bus bar, and is solidified. This solder-solidified condition is shown in FIG. 6 which is a cross-sectional view taken along the line VIxe2x80x94VI of FIG. 5. In FIG. 6, the showing of a board 29 (shown in FIG. 5) is omitted.
However, the solder 30 assumes a thin film-like configuration as a whole because of its wide wetting area, and will not form any solder fillet. As a result, there has been encountered a problem that the solder joint strength is insufficient, so that the reliability of the solder joint is lowered.
In order to solve this problem, there has been proposed a method in which spot-plating of metal, having excellent solder wetting properties, is applied to the soldering land 25a, 26a, 27a, 28a of each bus bar 25, 26, 27, 28 so that the wetting area of the soldering land 25a, 26a, 27a, 28a is limited to a narrow range. However, this method has a problem that the production efficiency is lowered, so that the production cost greatly increases.
This invention seeks to solve the above problems, and an object of the invention is to provide a structure of joining a chip part to bus bars, in which the sufficient solder joint strength is obtained without increasing the cost, while ensuring the production efficiency, so that the solder joint of high reliability can be achieved.
In order to solve the aforesaid object, the invention is characterized by having the following arrangement.
(1) A joining structure comprising:
two conductive bus bars spaced from each other;
a chip part including connecting portions for an electric circuit, mounted on the two bus bars;
recesses formed in soldering lands of the two bus bars, respectively, so that the connecting portions are spaced apart from inner edges of the recesses; and
a solder fillet formed between at least part of each of the recesses and the corresponding connecting portion.
(2) The joining structure according to (1), wherein the connecting portions has a flat plate-like shape on opposite side surfaces of the chip part, and the connecting portions form a part of a bottom surface of the chip part.
(3) The joining structure according to (1), wherein bottom surfaces of the connecting portions extend inwardly toward each other.
(4) The joining structure according to (1), wherein
each of the recesses includes a flat bottom surface, and
the solder fillets are formed in a state that lower surfaces of the connecting portions are held by the bottom surface.
(5) The joining structure according to (1), wherein
the solder fillets are formed in a state that the connecting portions are spaced from a bottom surfaces of the recesses, so that a solder layer is formed between the bottom surfaces and a lower surface of the connecting portion.
(6) The joining structure according to (1), wherein
each of the recesses includes a projection platform having a height lower than an upper surface of the bus bars, and
the solder fillets are formed in a state that lower surfaces of the connecting portions are held by the projection platforms, respectively.
(7) The joining structure according to (1), wherein the soldering lands correspond to a predetermined solder wetting area.